Fighting a difficult to detect foe: the swede midge

by Bill and Mary Weaver
The swede midge, well established in much of New York, including counties along the western PA border, has now been found in Ohio and this past summer was identified on an organic farm in Michigan. Scientist believe it’s only a matter of time until this tiny pest finds its way into all the crucifer growing areas of the U.S. It appears to be well adapted to do so.
Midge is an apt name for the tiny flying insect that lays about 100 eggs in its 1-5 day adult lifetime. The eggs then hatch into the critter that does the damage, a tiny maggot with a particular fondness for broccoli and cauliflower. It is happy to munch, however, on any cruciferous vegetable (including horseradish and canola), and doesn’t spurn cruciferous weeds, although it prefers the cultivated types when they are available.
The delicate, short-lived adults are little noticed. The maggots are also likely to go unnoticed, until you have a bad infestation built up, and you begin to see troubling symptoms on your vegetables that make them unmarketable: puckered leaves; “blind” heads or small, multiple heads on your broccoli, cauliflower and cabbage; twisted plant parts; or brown, corky spots where the maggots have fed.
Trouble is, these same symptoms can be caused by other factors, including herbicide damage, mechanical injury from cultivation, injury by other insects and animals, molybdenum deficiency, heat or cold stress, or genetic variations. The only way to diagnose the maggot or its damage for sure is to catch the culprit in the act and send it to a diagnostic lab. If the maggot has already dropped into the ground to pupate (it can have up to five generations a year), it can be anyone’s guess which of the above factors caused the damage.
Scout your crucifer fields, checking growing tips and slightly older leaves for the maggot. A 10x hand lens makes them easier to spot. Look also for telltale puckering and crinkling of leaves with a sort of ‘drawstring bag effect’ and twisting, which unfortunately, can protect the maggot from the effects of contact sprays. The leaf symptoms are most noticeable on middle-aged leaves that were damaged by feeding when they were part of the growing tip. A give-away can be brownish, dry patches where the maggots fed, but these are not absolutely diagnostic.
The dry, corky scarred patches are frequently (not always) the eventual result of the toxic salivary fluids the maggot injects into the plant which enable it to suck cell liquids. These toxic secretions cause the plant symptom already mentioned. Needless to say, munched-on plants with these effects are likely to be unmarketable.
What’s a grower to do? First, be watchful. Add the above symptoms, plus the symptom of moist growing tips, to the list of items you look for when scouting your crucifers. If you see anything “interesting,” inspect it closely for tiny maggots, and if you find them, collect and send them for expert analysis.
It’s wise to scout frequently, because if these symptoms show up after the small maggots have (literally) thrown themselves from the plant to the ground to pupate, the diagnosis will be in question.
Crop rotation to non-crucifers can provide good control, especially in large fields. Adults emerging from the soil of last season’s crucifers will be looking for another crop on which to lay their eggs and will die if they only find a non-crucifer crop in the field. Even in smaller fields rotation will be helpful, and better if you rotate your crucifers more than 100 yards upwind from last year’s crucifer fields. The adult females are not strong fliers, and are short-lived. It is best if you can rotate former crucifer fields out of crucifers for at least two years, since there is evidence that some pupae do not emerge as adults until the second year.
Adult swede midges also tend to be found more commonly in hedgerows and at the edges of fields rather than in wide-open fields. Scout particularly carefully in crucifer fields near such areas.
Third, as soon as you have harvested your crucifer crops— or if a field is badly infested — chop, plow, or disc the plants under. This will kill maggots in the plants, as well as any living tissue on which adult females could lay their eggs. Side shoots produced after broccoli or cabbage is harvested are ideal egg laying sites, especially while the weather is warm.
Soil manipulation can help by varying the moisture level or depth of the pupae. This can affect their ability to emerge. Coverage with more than 5 cm. of soil greatly reduces emergence and delays emergence times. Flooding the fields to 100 percent moisture levels during non-crop periods can also cut emergence, but be careful about soil diseases.
Also, control cruciferous weeds. Although swede midges prefer crop plants, they can survive and reproduce during rotation years on cruciferous weeds, such as watercress and wild mustards, including shepherd’s purse.
Sticky traps and pheromone traps usually yield adult midges that are damaged enough so that positive identification of the swede midge adult cannot be made.
Cabbage and Brussels sprouts are less attractive to egg laying females than broccoli and cauliflower. Also, some varieties of broccoli, such as ’Everest’ and ‘Triathlon’ are less attractive for egg laying than ‘Paradox’. As far as is known, however, there are no cruciferous crops or varieties that are completely resistant.
A key for control is to start with clean transplants, either from an area that is free of Swede midges, or transplants that are certified to be clean. When hardening off young plants, cover with a floating row cover or netting to prevent egg laying if adult swede midges could be near your area.
There are sprays that are effective. Imidacloprid, for example, gave 3 to 5 weeks of control. In a greenhouse study at Cornell on cauliflower, Assail 30SG gave very effective foliar control, if coverage is good, regardless of plant size, but only for nine days.
“When pressure is high, you need continuous control,” explained Dr. Anthony Shelton, Professor of Entomology at Cornell. “The best control we have found,” he continued, “is to use a drench of Imidacloprid soon after transplanting, which should provide control for well over a month. Then follow up with sprays of a systemic insecticide, following label directions.”
The growers hardest hit will be organic growers with a small land base who grow a variety of crucifers for their customers. These growers can’t use systemics, so swede midge numbers can grow unchecked in successive generations. “There are currently no consistently effective controls that meet organic standards that are known or under study, although netting your acreage (cost +/-$4,000 per acre) has been used in New York by Christy Hoepting with good results. Work has been done with intercropping and plant extracts, but recommendation of any tactic right now for organic growers is premature,” stated Shelton. Making only one planting of crucifers, early in the season, might help to prevent problems.